The present invention relates to a receiving circuit, and more particularly, to a receiving circuit of an infrared data communication apparatus for a portable terminal, such as a personal digital assistant (PDA), or an electronic device, such a as a cellular phone.
FIG. 1 is a schematic block diagram of a prior art receiving circuit 10, which is employed in an optical communication apparatus.
The receiving circuit 10 includes a photodiode (PD) 11 and a light receiving amplifier 12. The light receiving amplifier 12 includes a preamplifier 13, a main amplifier 14, a comparator 15, a DC light cancel circuit 16, and a DC feedback (DCFB) circuit 17.
The photodiode 11 generates a receiving current IPD, which corresponds to the received light. The preamplifier 13 coverts the receiving current IPD to a voltage signal VA1. The main amplifier 14 amplifies the voltage signal VA1 and generates an amplified signal VA2. The comparator 15 generates a binary receiving signal RX from the amplified signal VA2 based on a threshold value voltage VTH.
The received light may include DC light, such as sunlight. DC light generates a direct current (DC) component of the receiving current IPD, which flows through the photodiode 11. The DC component has a frequency, which is lower than a predetermined frequency band that includes a communication frequency. The DC light cancel circuit 16 feeds back the receiving current IPD, or the current for canceling the DC component included in the voltage signal, to the preamplifier 13.
The DCFB circuit 17 is used to cancel the input offset of the main amplifier 14. The DCFB circuit 17 has a plus input terminal, to which the amplified signal VA2 of the main amplifier is applied, and a minus input terminal VA2, to which a reference voltage VREF is applied. The output terminal of the DCFB circuit 17 is connected to the minus input terminal of the main amplifier 14. The voltage signal VA1 is applied to the plus input terminal of the main amplifier 14. The reference voltage VREF is provided to the minus input terminal of the main amplifier 14 via a resistor RB. The DCFB circuit 17 generates current, which corresponds to the potential difference between the amplified signal VA2 and the reference voltage VREF. The output current of the DCFB circuit 17, the reference voltage VREF, and the resistance of the resistor RB determines the voltage that is supplied to the minus input terminal of the main amplifier 14. The DCFB circuit 17 outputs current so that the DC component of the voltage signal VA1 of the main amplifier 14 coincides with the voltage supplied to the minus input terminal. That is, the DCFB circuit 17 cancels the input offset voltage in the main amplifier 14.
The prior art light receiving amplifier 12 has the four shortcomings that are described below.
(1) The occurrence of an input/output wraparound phenomenon of the light receiving amplifier 12 results in erroneous functioning. The wraparound phenomenon occurs directly or through a power supply line between the digital output and analog input of the light receiving amplifier 12 and produces noise in the light receiving signal RX. The noise causes erroneous functioning in an internal circuit of the optical communication apparatus.
As shown in FIG. 2(a), the level of the receiving signal RX normally changes in accordance with a light input signal. As shown in FIG. 2(b), when the input/output wraparound phenomenon occurs, a spike current is generated if the level of the light receiving signal RX changes. The spike current acts as a switching noise and affects the input terminal or the PD 11. Thus, the amplified signal VA2 of the comparator 15 includes noise and unnecessary pulses (the pulses encircled by the dotted lines) that appear in the receiving signal RX. Further, in some cases, the input/output wraparound phenomenon may cause oscillation.
(2) A differential amplifier having a high gain is employed as the main amplifier 14. However, the input offset of the high gain amplifier is not uniform. Thus, as described above, the DCFB circuit 17 is provided to feed back the DC component and cancel the input offset of the main amplifier 14. However, the amount of feedback of the DCFB circuit 17 is easily changed in accordance with the level of the voltage signal VA1 in the main amplifier 14 or the duty ratio of the main amplifier 14.
(3) When using a device incorporating the optical communication apparatus outdoor, the DC components included in the receiving circuit IPD increases. This increases the amount of the DC components cancelled by the DC light cancel circuit 16 and increases noise. This decreases the receiving sensitivity.
(4) When optical communication is performed at a close distance, a large amount of light is irradiated to the PD 11. This causes the generated receiving signal RX to have a pulse width that does not comply with the regulated standard (i.e., the pulse width is greater than the regulated width).